Objective: To investigate and control an outbreak of carbon dioxide (CO2) dependent meticillin resistant Staphylococcus aureus (MRSA) in a regional liver unit (RLU).

Methods: On 15th March 2008 a liver transplant recipient was screened for MRSA on admission to the critical care unit from the RLU. Small poorly-growing green colonies were isolated from a nasal swab on chromogenic MRSA identification media (chromID MRSA, bioMérieux) after 24 hours aerobic incubation. They were slide-coagulase positive (Slidex Staph-Plus reagent, bioMérieux). The strain repeatedly failed to grow after 24 and 48 hours aerobic incubation on Isosensitest agar (IST, Oxoid) or 5% horse blood agar (BA, TSC) for susceptibility testing. When subcultured onto BA and incubated overnight at 37oC in 5% CO2, however, a heavy growth of an isolate with colonial morphology typical of S.aur was seen. Susceptibility testing performed on IST according to British Society for Antimicrobial Chemotherapy guidelines but in CO2 enriched atmospheric conditions (5% CO2) confirmed that this was meticillin resistant S.aur (MRSA), also resistant to erythromycin, clindamycin, moxifloxacin and trimethoprim.

A possible outbreak was suspected as the medical microbiology/infection control teams were aware of another liver transplant recipient on the RLU who was found, in February 2008, to be colonised/infected with a strain of MRSA that grew much better in 5% CO2 than aerobically. All patients and staff on the RLU were screened for carriage of CO2 dependent MRSA using MRSA ID media in 5% CO2. A deep clean of the ward was carried out and infection prevention and control practices were reinforced.

Results: Four further cases (3 patients, 1 staff member) were found to be colonised at one or more sites by a CO2 dependent strain of MRSA. Ongoing targeted screening revealed a seventh case five weeks after the initial outbreak. This patient was admitted to the RLU during March 2008 but had been discharged two days prior to recognition of the outbreak and screening swabs were found to be positive when he was readmitted to the RLU in May.

Molecular analysis confirmed that all strains were identical: EMRSA-15 (ST22-SCCmecIV).

Conclusions: To our knowledge we report the first outbreak of CO2 dependent MRSA. Similar outbreaks may be missed if screening swabs are processed by conventional methods. Establishing the local prevalence of CO2 dependent MRSA is necessary to determine whether targeted screening is required.

Session Details

Date:

16/05/2009

Time:

00:00-00:00

Session name:

19th European Congress of Clinical Microbiology and Infectious Diseases